The House Republican plan to prevent, through the middle of next year, the looming insolvency of the Highway Trust Fund is drawing grumbles from both the left and the right, but there is increasing recognition that Congress has little choice but to enact it, or something like it.

Resignation that passing a short-term extension is likely the only way to avoid an August shutdown of transportation projects across the country was on full display Wednesday, the eve of a markup of the new proposal in the House Ways and Means Committee.

"It's the only proposal out there," Chairman Dave Camp, R-Mich., told reporters Wednesday afternoon in defense of his bill.

It's the kind of legislation that could only have a chance of passage under the circumstances in which lawmakers now find themselves: The waning days before the August recess, when the coffers of the Highway Trust Fund will drop to the lowest sustainable level.

Since the release of Camp's proposal on Tuesday, many Republicans in both chambers have slammed it for using a decade’s worth of savings and revenue to pay for less than a year of spending.

“It’s generational theft and it’s an embarrassment to our country,” said Sen. Bob Corker, R-Tenn., adding he would vote against the bill if it came before the Senate. “I realize that for the fifth time since 208 both bodies will show political cowardice and do what’s getting ready to happen but I hope that part of that will be laying out a vision toward doing something longer-term.”

Meanwhile, Democrats aren't happy that it extends the transportation project funding program through May 31 of next year, putting off difficult long-term decisions that they believe should be resolved now.

"It slides 'til next May, which means it will slide maybe for another year, or two, or three, so that, I think, is not particularly responsible," said Rep. Earl Blumenauer, D-Ore., a member of the Ways and Means Committee who also takes a keen interest in transportation policy.

That sentiment is shared by Camp's Democratic counterpart, ranking member Sander M. Levin of Michigan, who said he wanted a six-year extension enacted this year. But he, like Blumenauer, wouldn't say how he would vote at the Thursday markup.

Party leaders have been especially careful not to come out swinging against a proposal they might otherwise be inclined to snub, for fiscal and logistical reasons. House Speaker John A. Boehner, R-Ohio, called the proposal “solid” on Wednesday morning and said he looked forward to a vote on the House floor next week. And Minority Whip Steny H. Hoyer, D-Md., would only say that he hoped the bill would remain bipartisan throughout the legislative process.

In addition to wanting to keep their powder dry, they also might be hoping to avoid upsetting the delicate cross-chamber, cross-party talks that have been underway for more than a week.

A Senate Democratic aide actually said Democrats were encouraged by Camp’s opening offer.

“It’s certainly not kowtowing to the tea party that we’ve seen in the past,” the aide said. “There’s some evidence that they’re at least coming to the table a bit.”

Camp's plan would rely on a pension accounting maneuver known as “pension smoothing,” an extension of customs duties and a transfer of money from a separate trust fund designed to clean up underground oil spills to come up with the roughly $10 billion needed to keep the transportation program afloat. He told reporters he was confident that a "bipartisan, bicameral bill" would emerge before the August recess that resembles what he's put on the table.

Ways and Means Republican Dave Reichert, R-Wash., predicted the bill was likely to pick up some Democratic support and lose some Republican votes, but that ultimately it would go to the House floor next week and pass.

“That’s just the kind of environment we live in today,” he said.

Aides in both parties said Senate Finance Committee leaders continue to work on their own plan to keep the trust fund solvent beyond the Aug. 1 deadline that would rely on a roughly similar funding mechanism, though Senate negotiators have yet to agree on how long to extend the program. Senate Finance Committee Chairman Ron Wyden, D-Ore., has said he wants the short-term patch to last only until the end of December but Republicans are pushing for a longer measure.

The timing matters in light of November’s midterm elections. Both sides realize that the short-term bill now under discussion is only a prelude to a broader, contentious debate over how to fund infrastructure projects for the next six years.

An extension that expires before the end of 2014 guarantees that Democrats will still control the Senate and play a major role in the talks. A longer extension, however, risks turning the process over to Republicans who could find themselves in control of both chambers should they win the Senate in November.

House Transportation and Infrastructure Chairman Bill Shuster, R-Pa., said the longer patch as envisioned by Camp would allow states to begin planning for the 2015 summer construction season.

“If you end it in December, you screw up the planning process,” he said. “You screw up the next construction season.”

Camp on Tuesday called the shorter extension a “ploy” by Democrats to push through a gas tax increase at the end of the year.

On Wednesday, Sens. Tom Carper of Delaware and Barbara Boxer of California, both Democrats, chose to strike back, saying a longer patch would be “a disaster.”

“You’re talking about kicking the responsibility out from this Congress to the next Congress and we don’t even know at that time what other issues will be coming up,” Boxer said.